1. Field of the Invention
The present invention relates to a method of manufacturing a membrane electrode assembly.
2. Description of Related Art
A membrane electrode assembly (MEA) used in a fuel cell is a power generation unit in which an electrode catalyst layer is formed on both surfaces of an electrolyte membrane. Various methods for manufacturing this membrane electrode assembly have been disclosed.
For example, Japanese Patent Application Publication No. 2004-288391 (JP 2004-288391 A) describes a method of manufacturing a membrane electrode assembly, the method including: preparing a transfer sheet on which an electrode catalyst layer is formed by applying a catalyst ink to the transfer sheet; drying the prepared transfer sheet to remove a solvent; performing a heat treatment on the dried transfer sheet; and transferring the heat-treated transfer sheet to an electrolyte membrane.
Japanese Patent Application Publication No. 2012-209268 (JP 2012-209268 A) describes a method including: forming an electrode catalyst layer by applying a catalyst layer-forming mixture (catalyst ink) to an electrolyte membrane and drying the catalyst layer-forming mixture; and performing a heat treatment on the formed electrode catalyst layer in a temperature range of 40° C. to a glass transition temperature of an electrolyte resin (cation exchange resin, ionomer), which is contained in the electrode catalyst layer, in order to remove organic materials such as alcohol from the electrode catalyst layer.
Japanese Patent Application Publication No. 7-29576 (JP 7-29576 A) describes a method including: drying an alcohol solvent to be removed from a catalyst paste when the catalyst paste is applied to a gas supply layer (porous sheet) and dried.
Japanese Patent Application Publication No. 2014-60167 (JP 2014-60167 A) describes a method including: preparing a catalyst layer transfer sheet on which an electrode catalyst layer is formed by applying a catalyst paste to a substrate sheet and drying the catalyst paste; and maintaining a high-temperature state of a glass transition temperature of the electrolyte membrane±50° C. when the electrode catalyst layer is transferred to a surface of the electrolyte membrane to peel the substrate sheet by applying pressure to the electrolyte membrane while heating the catalyst layer transfer sheet, thereby suppressing peeling defects of the electrode catalyst layer.
Japanese Patent Application Publication No. 2010-61865 OP 2010-61865 A) describes a method including: preparing a substrate sheet on which an electrode catalyst layer is formed by applying an ink composition (catalyst ink) to the substrate sheet and drying the ink composition; and performing a heat treatment on the formed electrode catalyst layer at a temperature which is higher than a glass transition temperature of an ionomer (electrolyte resin, ion exchange resin) in the electrode catalyst layer, thereby improving the strength of a structure of the electrode catalyst layer.
Japanese Patent Application Publication No. 2005-50734 (JP 2005-50734 A) discloses a method including: performing a heat treatment on a sheet of a mixture of carbon particles and a cation exchange resin (electrolyte resin, ionomer) in a temperature range of a glass transition temperature of the cation exchange resin to a decomposition temperature thereof, thereby improving the strength of a structure of a catalyst electrode layer, for example, improving the stability of a crystal structure of the cation exchange resin or the adhesion between surfaces of the carbon particles and the cation exchange resin.
The present inventors found the following problems regarding the related art. That is, for example, in JP 2004-288391 A, alcohol gas is produced by evaporating alcohol (for example, ethanol or propanol) as a solvent during the drying of the transfer sheet. When the heat treatment is performed on the electrode catalyst layer in an environment in which the produced alcohol gas remains, the alcohol gas is oxidized through an oxidation reaction caused by the catalyst in the electrode catalyst layer. For example, ethanol is converted into acetic acid, and oxidation heat is generated. The generation of oxidation heat causes the thermal decomposition of the ionomer in the electrode catalyst layer. For example, when the ionomer is a fluororesin (for example, “Nafion” (trade name)) which is a polymer having a sulfonic acid group (—SO3H) at a terminal group thereof, the amount of sulfate ions (So42−) contained in the electrode catalyst layer increases by the sulfonic acid group being thermally decomposed due to the oxidation heat. An increase in the amount of sulfate ions contained in the electrode catalyst layer decreases pH in a cell of a fuel cell, more specifically, pH in a membrane electrode assembly constituting the cell and causes the environment to be acidic, and poisoning of the electrode catalyst layer occurs. As a result, a decrease in the proton conductivity (cation conductivity) of the electrode catalyst layer, an increase in the impedance of an electrode including the electrode catalyst layer and a gas diffusion layer, or a decrease in the output of power generation of a fuel cell may occur.
The techniques disclosed in JP 2004-288391 A, JP 2012-209268 A, JP 7-29576 A, JP 2014-60167 A, JP 2010-61865 A, and JP 2005-50734 A do not describe the following point that: sulfate ions produced in the process of forming the electrode catalyst layer cause poisoning of the electrode catalyst layer even in the initial stage of a fuel cell (membrane electrode assembly), which may cause a decrease in the proton conductivity of the electrode catalyst layer, an increase in the impedance of an electrode including the electrode catalyst layer and a gas diffusion layer, or a decrease in the output of power generation of a fuel cell.